Method for under-pressure carburizing of steel workpieces

ABSTRACT

The subject of this invention relates to method carburizing of steel products, mainly parts of machines, vehicles and every mechanical apparatus, in vacuum furnaces under reduced pressure and elevated temperature. The method of under-pressure carburizing of steel workpieces according to present invention relates to introduction of active nitrogen carrier during heating up of the load. Introduction of the active nitrogen carrier is terminated when the load reaches temperature required to start carburizing process; from this temperature the carbon carrier is added. Pressure in the furnace chamber during continuous or pulse introduction of the active nitrogen carrier should be maintained within the ranges from 1 to 500 mbar.

The object of this invention relates to the method for carburizing of steel products, mainly parts of machines, vehicles and all types of mechanical apparatuses, in vacuum furnaces under reduced pressure and elevated temperature.

A method for carburizing of products made of steel in a furnace chamber is known from the U.S. Pat. No. 6,187,111. In this method, vacuum in the range of 1 to 10 hPa is generated and the temperature of the carburizing process is maintained between 900° C. and 1100° C. The carbon carrier there is gaseous ethylene. Another U.S. Pat No. 5,205,873, describes the carburizing process carried out under low pressure in a furnace chamber heated up to temperatures between 820° C. and 1100° C. This process starts in a chamber where an initial vacuum up to 10⁻¹ hPa was generated to remove the air. Then, after backfill of the chamber with pure nitrogen, workpieces to be carburized are placed into it. In the loaded chamber, a vacuum in the range of 10⁻² hPa is generated and the charge is heated up to the austenitizing temperature and this temperature is maintained until the temperatures across the workpiece are equalised; afterwards the furnace chamber is backfilled with hydrogen up to 500 hPa. Then ethylene as the carbon carrier is introduced under the pressure from 10 to 100 hPa and a gas mixture consisting of hydrogen and ethylene is created, in which the ethylene content ranges from 2% to 60% of the gas mixture by volume.

Also the U.S. Pat. No. 5,702,540, describes the method of carburizing, according to which the charge is pre-heated under vacuum and gaseous unsaturated aliphatic hydrocarbons are used as the carbon carrier. This method can also be applied for carbonitriding, where together with the carbon carrier an active nitrogen carrier is introduced to the furnace chamber.

The method for under-pressure carburizing of steel workpieces according to the present invention consists in the introduction of ammonia into a vacuum furnace chamber at the moment when the charge reaches the temperature of 400° C. and it is introduced into the vacuum furnace chamber until the charge reaches the temperature required for start of the carburizing process, which is the moment when the carbon carrier is started to be introduced.

The method according to the present invention is distinguished by a possibility of an effective application of the upper range of carburizing temperatures due to restraining the growth of austenite grains as a result of initial saturation of the surface area with nitrogen, without the formation of unfavorable nitrides on the charge surface, and in consequence the process is significantly accelerated.

One of possible implementations of the method for under-pressure carburizing of steel workpieces according to the present invention is illustrated by the following examples:

EXAMPLE 1

A furnace chamber of the size 200×200×400 mm was loaded with workpieces made of low carbon steel grades C15, 16CrMn5 and 17CrNiMo. The total surface area of the charge was 0.4 m². After pre-heating under vacuum up to 400° C. ammonia was introduced to the furnace chamber interior with a constant flow rate of 50 l/hr. The process atmosphere was maintained under a constant pressure of 5 mbar. When steel workpieces had reached the temperature of 950° C., the introduction of ammonia was interrupted, and carburizing atmosphere was introduced for twenty minutes and a constant temperature of the vacuum furnace chamber was maintained; the atmosphere was made up of the carbon carrier in the form of a mixture of ethylene and acetylene in the volume ratio 1, mixed with hydrogen in the volume ratio 1.17, introduced with a constant flow rate 190 l/hr and thus generating pressure pulse in the furnace chamber within the range of 3 to 8 mbar. For the next 8 minutes steel workpieces were heated under vacuum at the temperature of 950° C. and then slowly cooled under vacuum down to the ambient temperature. On individual steel workpieces carburized layers were produced with the following performance. Case depth to limit structure- 50% perlite + 50% Surface carbon austenite Original grain Steel grade concentration [%] [mm] size [mm] C15 0.65 0.40 ± 0.005 40%-0.008 60%-0.011 l6CrMn5 0.71 0.46 ± 0.005 50%-0.011 50%-0.013 l7CrNiMo 0.72 0.47 ± 0.005 70%-0.011 300%-0.016  The surface of all workpieces after carburizing was clean and bright without any evidence of soot and tar.

EXAMPLE 2

A furnace chamber of the size 200×200×400 mm was loaded with workpieces made of low carbon steel grades 16CrMn5 and 17CrNiMo. The total surface area of the load was 0.4 m². After pre-heating under vacuum up to 400° C. ammonia was introduced to the furnace chamber interior with a constant flow rate of 50 l/hr. The process atmosphere was maintained under a constant pressure of 5 mbar. When steel workpieces had reached the temperature of 950° C., the introduction of ammonia was interrupted, and carburizing atmosphere was introduced for twenty minutes and a constant temperature of the vacuum furnace chamber was maintained; the atmosphere was made up of the carbon carrier in the form of a mixture of ethylene and acetylene in the volume ratio 1, mixed with hydrogen in the volume ratio 1.17 introduced with a constant flow rate 190 l/hr and thus generating pressure pulse in the furnace chamber within the range of 3 to 8 mbar.

For the next 20 minutes steel workpieces were heated under vacuum at the temperature of 950° C. and then fast cooled down to the ambient temperature under nitrogen at the pressure increased up to 6 bar. On individual steel workpieces carburized layers were produced with the following performance. Case depth Steel grade Surface hardness [HV₀₁] to limit hardness 500 HV₀₁ 16CrMn5 744 0.48 ± 0.005 l7CrNiMo 820 0.49 ± 0.005 The surface of all workpieces after carburizing was clean and bright without any evidence of soot and tar.

EXAMPLE 3

A furnace chamber of the size 200×200×400 mm was loaded with wbrkpieces made of low carbon steel grades C15, 16CrMn5 and 17CrNiMo. The total surface area of the load was 0.4 m². After pre-heating under vacuum up to 400° C. ammonia was introduced to the furnace chamber interior with a constant flow rate of 50 l/hr. The process atmosphere was maintained under a constant pressure of 5 mbar. When steel workpieces had reached the temperature of 1000° C., the introduction of ammonia was interrupted, and carburizing atmosphere was introduced for twenty minutes and a constant temperature of the vacuum furnace chamber was maintained; the atmosphere was made up of the carbon carrier in the form of a mixtre of ethylene and acetylene in the volume ratio 1, mixed with hydrogen in the volume ratio 1.17 introduced with a constant flow rate 270 l/hr and thus generating pressure pulse in the furnace chamber within the range of 3 to 8 mbar. For the next five minutes steel workpieces were heated under vacuum at the temperature of 1000° C. and then slowly cooled under vacuum down to the ambient temperature. On individual steel workpieces carburized layers were produced with the following performance. Case depth to limit structure- 50% perlite + 50% Surface carbon austenite Original grain Steel grade concentration [%] [mm] size [mm] C15 0.66 0.52 ± 0.005 70%-0.011 30%-0.013 l6CrMn5 0.70 0.58 ± 0.005 50%-0.013 50%-0.016 17CrNiMo 0.70 0.59 ± 0.005 60%-0.013 40%-0.016 The surface of all workpieces after carburizing was clean and bright without any evidence of soot and tar. 

1. The method of under-pressure carburizing of steel workpieces with the introduction of the active nitrogen carrier to the vacuum furnace chamber, accompanied by the introduction of ammonia until the pressure of 1 to 500 mbar is reached is characteristic in that the ammonia is started to be introduced into the vacuum furnace chamber at the moment when the charge reaches the temperature of 400° C., and it is kept being introduced into the chamber until the moment when the batch reaches the temperature required for start of the carburizing process, i.e. the moment when the carbon carrier is started to be introduced. 